Technology update

Mar 13, 2014

Phonon polaritons propagate in 2D

To date, hexagonal boron nitride (a 2D layered material, also known as “white graphene”) has mainly been used as a substrate or as a spacer to make electronics devices and so-called van der Waals heterostructures. Now, researchers in the US have found that it can also act as a powerful waveguide in its own right and support propagating surface waves – or phonon polaritons. The result could be important for applications in high-density infrared data storage, information transfer and even ultrasound imaging on the nanoscale.

2D layered materials are creating a flurry of interest in labs around the world thanks to their unique electronic and mechanical properties that are very different from their 3D counterparts, which means that they might find use in a host of novel device applications. They are made up of individual atomic planes weakly held together by van der Waals forces. Most research in this field has focused on the most famous of 2D materials, graphene (a planar sheet of carbon atoms arranged in a honeycomb lattice), but researchers are now also beginning to turn their attention elsewhere.

Hexagonal boron nitride (hBN) is one such up and coming 2D compound and, like graphene, has excellent mechanical and thermal properties. Indeed, hBN has already proved itself to be a good substrate for graphene thanks to the fact that the two materials have very similar lattice constants.

Strong phonon resonances

hBN also boasts particularly strong phonon resonances that span a broad region of the technologically important IR band of the electromagnetic spectrum, which means that it could be ideal for use in optoelectronics devices. Phonons are quantized sound waves and some physicists believe that they could be used to transmit information in nanodevices if a suitable medium were to be found. They can be generated in nanoscale structures using laser light, and the wavelength of the phonons produced depends on the periodicity of the nanostructure in which they propagate.

hBN is especially interesting in this respect because its layer thickness can be controlled on the atomic scale – just like graphene’s. A team led by Dimitri Basov at the University of California, San Diego, is now saying that phonon polariton waves (which have a much shorter wavelength than light waves) can be “tuned” to particular frequencies and amplitudes by varying the number of layers in hBN.

First phonon polaritons in a van der Waals crystal

Phonon polaritons are collective resonant modes that are created by coupling between photons with optical phonons in polar dielectric crystals. Until now, phonon polaritons had only been studied in bulk materials such as silicon dioxide and silicon carbide, and this is the first time they have ever been observed in atomically thin crystals, says team member Siyuan Dai.

“Our work proves that these polaritons waves can now exist on the nanoscale,” he told nanotechweb.org. “This result means that they can be used to transmit information across such tiny distances and ultimately in nanodevices.” The phonons produced typically have wavelengths of tens of nanometres, which could also make them useful for applications such as non-invasive ultrasound imaging – the nano version of medical ultrasound.

Basov and colleagues obtained their result by illuminating hBN with infrared light, which launched phonon polaritons across the material surface. The researchers did this by focusing an infrared laser beam on the tip of an atomic microscope as it scanned across the hBN. “The propagating waves reflected at the edge of the material return to the tip and interfere with newly launched waves,” explained Dai. “As we scan the tip towards the edge, we are able to image these standing waves, which show up as interference patterns.” The amplitude and frequency of the waves depends on the number of layers in the hBN crystal.

As well as being useful in high-density infrared data storage, high-resolution ultrasound microscopy and energy or information transfer, the hBN polaritons might also be legitimate candidates for any application previously proposed for graphene plasmons, such as sensors and nanoscale circuits, adds Dai.